General flat cathode ray tubes are minutely described in U.S. Pat. Nos. 4,695,761, 4,710,670, and 4,716,334. The common characteristic of these flat cathode ray tubes is that a foil type shadow mask of the flat faceplate is fixed by a substantially rectangular rim which is bonded to the periphery of the inner surface of the faceplate.
In order to weld the shadow mask to the rim of the flat cathode ray tube, the shadow mask is mounted to an apparatus for tensely stretching the peripheral portion of the shadow mask by extension force applied by the apparatus. Under the condition that the shadow mask is tensely stretched, the shadow mask is welded to the rim which has been bonded to the faceplate.
An example of the aforesaid shadow mask stretching apparatus is illustrated in FIG. 1A and FIG. 1B, and the related art is described in U.S. Pat. No. 4,790,789.
The conventional stretching apparatus 10 includes a rectangular space 12 confined by a frame 11, and a clamping means 15 of a shadow mask 30 formed at the periphery of the frame 11. The clamping means 15 consists of clamping channel 16 and a clamping clip 17 for being fitted into the clamping channel 16. A pair of opposite jaws 16a are formed at the upper portion of the clamping channel 16, and the clamping clip 17 to be coupled with the channel 16 is a plate bent to be in ".OMEGA." shape.
The process for fixing the shadow mask 30 to the conventional shadow mask stretching apparatus is as follows.
The shadow mask 30 is placed on the stretching apparatus 10. Then, a pair of platens having a heater therein, which is described in U.S. Pat. No. 4,722,238 for example, are employed for heating the shadow mask 30.
When the shadow mask 30 is sufficiently heated to undergo thermal expansion to a certain extent, a plurality of the clips 17 as a coupling means are respectively inserted into each channel 16 formed in the frame 11 of the stretching apparatus, so that the periphery of the foil type shadow mask 30 is coupled to the frame 11 of the stretching apparatus by the clips 17, as shown in FIG. 1B.
The shadow mask 30 mounted to the stretching apparatus 10 is tensed by cooling to make the periphery of the shadow mask 30 be tightly strained against the stretching apparatus 10 under a constant tensile force.
While the shadow mask is mounted to the stretching apparatus, the faceplate corresponding to the shadow mask is paired together to be processed in a manufacturing process of a phosphor layer.
After the process for forming the phosphor layer on the faceplate is completed, the shadow mask mounted to the stretching apparatus is welded to the rim which is bonded to the faceplate. The shadow mask is then separated from the stretching apparatus, thereby finishing the mounting of the shadow mask to the faceplate.
As shown in FIG. 2, the disadvantages of the conventional stretching apparatus 10 which is employed in the aforesaid process are that the frame 11 of the stretching apparatus is in a rectangular shape, and the clamping means provided thereto is also in a rectangular shape, so that the tensile forces at the center portion and at the corner portion of each side are different from each other. That is, the expansions of the shadow mask along the diagonal direction and along the parallel direction of each side are different from each other by the initial tensile force applied to the shadow mask upon cooling, and the tensile force applied along each direction of the shadow mask is accordingly different from each other. Therefore, the shadow mask is abnormally expanded to result in undesirable deformation of the electron beam passing apertures formed in the shadow mask, or, wrinkling of the shadow mask at the corner portions thereof in some cases. Consequently, the quality of the product is deteriorated, and a realization of a high definition image of television becomes difficult.